Do scientists disagree about global warming?

Climate change has become a touchy subject in the Republican primary. Though some candidates oncesupportedplans to reduce carbon emissions, such strategies have fallen out of favor with Republicans in recent years. Even acknowledging that human beings are causing climate change can be politically problematic for some Republicans.

Our colleagues at the Miami Herald asked Republican presidential candidate and former Minnesota Gov. Tim Pawlenty about his views on climate change in an interview on Aug. 3, 2011. His response piqued our interest:

"Well, there’s definitely climate change. The more interesting question is how much is a result of natural causes and how much, if any, is attributable to human behavior. And that’s what the scientific dispute is about," said Pawlenty. "It’s something we have to look to the science on. The weight of the evidence is that most of it, maybe all of it, is because of natural causes... There’s lots of layers to it. But at least as to any potential man-made contribution to it, it’s fair to say the science is in dispute."

• The science about the causes of global climate change is in dispute.

To check the first claim, we turned to the most recent report by the U.N. Intergovernmental Panel on Climate Change (IPCC), a scientific body considered the leading international organization on climate science. "Most of the observed increase in global average temperatures since the mid-20th century is very likely due to the observed increase in anthropogenic greenhouse gas concentrations," the 2007 report states. "The observed widespread warming of the atmosphere and ocean, together with ice mass loss, support the conclusion that it is extremely unlikely that global climate change of the past 50 years can be explained without external forcing, and very likely that it is not due to known natural causes alone." (External forcing refers to anything that changes the climate that is outside of the normal climate system.)

In the United States, the U.S. Global Change Research Program coordinates and integrates federal research on climate. Its 2009 report mirrored the IPCC’s conclusions: "Observations show that warming of the climate is unequivocal. The global warming observed over the past 50 years is due primarily to human-induced emissions of heat-trapping gases. These emissions come mainly from the burning of fossil fuels (coal, oil, and gas), with important contributions from the clearing of forests, agricultural practices, and other activities."

The IPCC report states that the period from 1995 to 2006 contains 11 of the 12 warmest years on record since instrumental measurement began in 1850. It concluded that global surface temperature rose 0.76 degrees Celsius from the end of the 19th century (1850-1899) to the beginning of the 21st (2001-2005). The U.S. Global Change Research Program’s report similarly found that the average global temperature increase by 1.5 degrees Fahrenheit since 1900.

While these temperature changes may sound small, keep in mind that these are averages. The rise in temperature, and its visible effects, are more pronounced in certain parts of the world than in others. The polar ice sheets are particularly vulnerable to temperature increases. Greenland lost 36 to 60 cubic miles of ice from 2002 to 2006 while Antarctica lost 36 cubic miles of ice during roughly the same time period.

Brian Soden, a professor of meteorology and physical oceanography at the University of Miami, was one of the many scientific contributors to the 2007 IPCC report.

"The rise in global mean temperature since the 19th century may seem small, and projections of global mean warming over the next century range from 2 to 5 degrees Celsius," Soden said. "One way to put these numbers into perspective is to realize that the current climate is now only about 5 degrees Celsius warmer than it was during the last ‘glacial maximum’ (approximately 20,000 years ago) when ice covered much of North America extending all the way down to St Louis."

If you’ve been following the debate recently, you may remember that climate research on temperature was called into question in 2009 when stolen e-mails from the Climatic Research United at the University of East Anglia were released on the Internet. Global warming skeptics said the e-mails showed climate researchers were manipulating data.

But several inquiries debunked those allegations, including those conducted by the British Parliament, the U.S. Department of Commerce, Pennsylvania State University, and the InterAcademy Council. The inquiries found that while the scientists had made rude remarks about people who questioned climate change, they were not falsifying data. A few reports recommended greater transparency and sharing of climatic data, but the independent investigations exonerated the researchers of falsifying data.

We looked into the work of the most prominent and best credentialed people who have questioned the IPCC’s conclusions on global warming. Generally speaking, even these scientists do not claim that humans are making no contribution at all to rising temperatures. Rather, they tend to make more nuanced points. They question whether carbon emissions alone are driving up temperatures, or whether other human activities contribute as well. They question whether extreme weather events such as storms or floods can be conclusively linked to rising temperatures. And, they question whether significant changes to public policy are necessary as a means of coping with rising temperatures.

For Pawlenty’s second claim -- that there is dispute about the causes of climate change -- we decided to look at the opinions of world scientists about the issue.

Several studies have attempted to quantify how much agreement there is among scientists when it comes to climate change.

A 2010 study published in the Proceedings of the National Academy of Sciences - the official publication of the United States National Academy of Sciences - found that out of 1,372 climate researchers surveyed, approximately 97 to 98 percent of those actively publishing in the field said they believe human beings are causing the climate change, which they term anthropogenic (i.e., man-made) climate change. It also concluded that "the relative climate expertise and
scientiﬁc prominence" of the researchers unconvinced of man-made climate change are "substantially below that of the convinced researchers."

An earlier survey published in the 2009 issue of Eos -- a publication of the American Geophysical Union -- surveyed scientists from a wide range of disciplines (approximately 3,146) and asked: "Do you think human activity is a significant contributing factor in changing mean global temperatures?" Approximately 82 percent of the surveyed scientists answered yes to this question. Of those climate change specialists surveyed, 97.4 percent answered yes to this question.

Climate change skeptics have their own petition, commonly called the Oregon petition, that has been endorsed by 31,000 signers opposing restrictions on carbon emissions. But that petition has been criticized for not checking the credentials of its signatories or proving that the signatories exist.

To be clear, we’re not saying that no scientists dispute man-made climate change. When we contacted the Pawlenty campaign, they pointed us to several published pieces, such as writings by John R. Christy, who was counted as a dissenting researcher as part of the 2010 study from the Proceedings of the National Academy of Sciences.

But these skeptics seem to be a small -- even tiny -- minority, in contrast to Pawlenty’s comments suggesting significant disagreement. Many of the skeptics agree that climate change is occurring and that human activities play a part. But they disagree with some of the conclusions formulated in mainstream climate science.

To summarize: Based on our research, there is very little dispute in the scientific community, especially among climate specialists, on whether climate change is primarily caused by natural or man-made forces. The overwhelming majority of scientists polled feel that human activity is the primary driver of climate change. Also, based on scientific studies by the IPCC and others, global warming over the past 50 years has been primarily driven by human activity.

Based upon the preponderance of evidence we conclude that Tim Pawlenty’s claims are both incorrect and misleading to the public, who may not be familiar with the science behind climate change. It is not "fair to say the science is in dispute," as if there are good arguments on both sides. Rather, there is significant scientific consensus that human beings are contributing to global warming. We rate his statement False.

Editor's Note: Pawlenty announced he was dropping out of the presidential race on Aug. 14. Also, this report was changed to reflect a more expansive definition of external forcing.